MPTCP F. Song
Internet Draft H. Zhang
Intended status: Informational Beijing Jiaotong University
Expires: June 14, 2018 H. Chan
A. Wei
Huawei Technologies
Dec 13, 2017
One Way Latency Considerations for MPTCP
draft-song-mptcp-owl-03
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Abstract
This document discusses the use of One Way Latency (OWL) for
enhancing multipath TCP (MPTCP). Several use cases of OWL, such as
retransmission policy and crucial data scheduling are analyzed. Two
kinds of OWL measurement approaches are also provided and compared.
More explorations related with OWL will be helpful to the
performance of MPTCP.
Table of Contents
1. Introduction ................................................ 2
2. Conventions and Terminology.................................. 3
3. Potential Usages of OWL in MPTCP............................. 3
3.1. Crucial Data Scheduling................................. 4
3.2. Congestion control...................................... 5
3.3. Packet Retransmission................................... 6
3.4. Bandwidth Estimation.................................... 6
3.5. Shared Bottleneck Detection............................. 7
4. OWL Measurements in TCP...................................... 7
5. Security Considerations...................................... 8
6. IANA Considerations ......................................... 8
7. References .................................................. 8
7.1. Normative References.................................... 8
7.2. Informative Reference................................... 8
Authors' Addresses ............................................. 9
1. Introduction
Both end hosts and the intermediate devices in the Internet have
basically been equipped with more and more physical network
interfaces. The importance of interfaces, which had been widely used
in packet forwarding at the end hosts, had been confirmed and
utilized [RFC6419]. Moreover, to aggregate more bandwidths, to
decrease packet delay and to provide better services, the increased
capacity provided by the multiple paths created by multiple
interfaces is leveraged. Unlike traditional TCP [RFC0793], many
transport layer protocols, such as MPTCP [RFC6182] [RFC6824] enable
the end hosts to concurrently transfer data on top of multiple paths
to greatly increase the overall throughput.
Round-trip time (RTT) is commonly used in congestion control and
loss recovery mechanism for data transmission. Yet the key issue for
data transmission is simply the delay of the data transmission along
a path which does not include the return. It may be very different
of the latency for uplink and downlink between two peers. Latency in
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the opposite direction along a path can easily influence RTT, which
cannot accurately reflect the delay of the data transmission along
that path. Therefore, the use of One Way Latency (OWL) is proposed
to describe the exact latency from the time that data is sent to the
time data is received.
The performance of current practices of MPTCP can be further
improved by fully taking advantage of One Way Latency (OWL) during
the transmission is explained in this document. It may be asymmetric
of the OWL components in the forward and reverse directions of a RTT
so that it can provide a better measure to the user such as for
congestion control even with the regular TCP. It will be more
benefits when there are multiple paths to choose from.
This document discusses the necessary considerations of OWL in MPTCP.
The structure of this document is as follows: Firstly, it analyzed
several use cases of OWL in MPTCP. Secondly, two kinds of OWL
measurements are listed and compared. The considerations related
with security and IANA are given at the end.
The application programmer whose products may significantly benefit
from MPTCP will be the potential targeted audience of this document.
The necessary information for the developers of MPTCP to implement
the new version API into the TCP/IP network stack is also provided
in this document.
2. Conventions and Terminology
The key words "MUST", "MUST NOT", "GLUIRED", "SHALL","SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
One Way Latency (OWL): the propagation delay between a sender and a
receiver from the time a signal is sent to the time the signal is
received.
3. Potential Usages of OWL in MPTCP
There are a number of OWL use cases when MPTCP is enabled by the
sender and receiver. Although, only 5 use cases are illustrated in
this document, more explorations are still needed.
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3.1. Crucial Data Scheduling
During a transmission process, some crucial data often need to be
immediately sent to the destination. Examples of such data include
the key frame of multimedia and high priority chunk of emergency
communication. No one can guarantee the arrival sequence by using
the RTTs alone of the multiple paths.
The data rate in any given link can be asymmetric. In addition,
according to the amount of packet queue, the delay in a given
direction can change. Therefore, the same as that in the reverse
direction as exemplified in Figure 1, delay in a forward direction
in a path is not necessary.
-------- OWL(s-to-c,path1)=16ms OWL(c-to-s,path1)= 5ms ----- |
| / RTT(path1)=21ms \ |
| | | |
+------+ +------+
| |-----> OWL(c-to-s,path2)= 8ms -----| |
|Client| |Server|
| |----- OWL(s-to-c,path2)= 8ms OWL(c-to-s,path3)=10ms ----- |
\ /
-------- OWL(s-to-c,path3)= 8ms
Internet-Draft OWL Considerations for MPTCP Dec 2017
3.2. Congestion control
Congestion in a given direction does not necessarily imply
congestion also in the reverse direction.
-------- No congestion (path 1) Congestion (path 1) ----- |
| / \ |
| | | |
+------+ +------+
|Client| |Server|
+------+ +------+
| | | |
| \ / |
| -----> No congestion (path 2) ----- |
\ /
-------- Congestion (path 2)
Internet-Draft OWL Considerations for MPTCP Dec 2017
3.3. Packet Retransmission
Continuous Multipath Transmission (CMT) increases throughput by
concurrently transferring new data from a source to a destination
host via multiple paths. However, the sender needs to select a
suitable path for retransmission, when packet is identified as lost
by triple duplicated acknowledgements or timeout. Outstanding
packets on multiple paths may reach to the destination disorderly
and trigger Receive Buffer Blocking (RBB) problem (Figure 3) which
will further affect the transmission performance, due to the popular
mechanisms of sequence control in reliable transport protocols.
Packetwith octets sequence # 0- 499(lost)
---> Packetwith octets sequence #1000-1499(rcvd) ------
/ Packetwith octets sequence #2000-2499(rcvd) \
| |
+------+ +--------+
|Sender| |Receiver|
+------+ +--------+
| |
\ Packetwith octets sequence # 501- 999(lost) /
-----> Packetwith octets sequence #1501-1999(lost) -----
Packetwith octets sequence #2501-2999(lost)
Figure 3. Example of Receive Buffer Blocking: The packet containing
octets 0-499 is lost. On the other hand the packets containing
Octets 500-999, 1000-1499, 1500-1999, 2000-2499, and 2500-2999 have
all been received. The octets 500-2999 are then all buffered at the
receiver, and are blocked by the missing octets 0-499.
The sender can quickly determine the specific path with minimum
latency in the forward direction by using the results of OWL
measurement. As soon as the receiver obtains the most needed packet
(s), RBB can be relieved and be submitted to the upper layer.
3.4. Bandwidth Estimation
It's beneficial to understand the bandwidth condition for data
packet scheduling, and load balancing, etc. OWL could be integrated
with bandwidth estimation approaches without interrupting the
regular transmission of packets.
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3.5. Shared Bottleneck Detection
Fairness is critical especially when MPTCP and ordinary TCP coexist
in the same network. OWL measurements can be treated by sender as
the sample process of shared bottleneck detection, and sender adjust
the volume of data packet on multiple paths accordingly.
4. OWL Measurements in TCP
The timestamp option in TCP [RFC7323] may be invoked to estimate
latency. The time (TSval) of sending the data is provided in the
option when sending data. The receiver acknowledges the receipt of
this data by echoing this time (TSecr), and also the time (TSval) of
sending this acknowledgment is provided. Although there are two
problems, the difference of these times in the acknowledgment of
data from the sender can help to estimate the OWL from the sender to
the receiver.
First, there may be delay from the time the receiver who has
received the data to the time when the acknowledgment is sent. Then,
the above number may be an upper bound of OWL.
Second, the clocks between the sender and the receiver may not be
synchronized. The OWL can be showed in different paths by the above
measure only if the clocks synchronized. The comparison of OWLs
among different paths is limited to showing the OWL differences
among them without clock synchronization.
Two kinds of OWL measurement approaches are available: absolute
value measurement and relative value measurement.
In order to obtain the absolute value of OWL, the primary condition
of measurement is clock synchronization. End hosts can calibrate the
local clock with the remote NTP server by using network time
protocol (NTP) [RFC5905]. The additional information or optional
capabilities can even be added via extension fields in the standard
NTP header [RFC7822]. The calibration accuracy can reach to the
millisecond level in less congested situations. The obvious burden
here is to persuade the end hosts to initialize the NTP option.
It's more than enough to obtain the relative value of OWL in some
circumstances to establish applications on top of it. For example,
the sender may only care about which path has the minimum forwarding
latency when retransmission is needed. When bandwidth is being
estimated, the difference of forward latency, i.e. delta latency,
among all available paths is needed. Both sides could obtain the
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relative value of OWL by exchanging with correspondent end host the
local timestamps of receiving and sending the packets.
The overheads are the extra protocol requirement and synchronization
accuracy, while absolute value measurement of OWL is more convenient
for the applications. On the contrary, it's no need for relative
value to worry about the accuracy whereas the overhead is to add
timestamps into the original protocol stack.
5. Security Considerations
This document does not contain any security considerations. However,
the relevant mechanisms definitely need to be established by future
applications of OWL in MPTCP to improve security.
6. IANA Considerations
This document presents no IANA considerations.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI
10.17487/RFC2119, March 1997, .
7.2. Informative Reference
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, DOI 10.17487/RFC0793, September 1981,
.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
.
[RFC6182] Ford, A., Raiciu, C., Handley, M., Barre, S., and J.
Iyengar, "Architectural Guidelines for Multipath TCP
Development", RFC 6182, DOI 10.17487/RFC6182, March 2011,
.
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[RFC6356] Raiciu, C., Handley, M., and D. Wischik, "Coupled
Congestion Control for Multipath Transport Protocols", RFC
6356, DOI 10.17487/RFC6356, October 2011, .
[RFC6419] Wasserman, M. and P. Seite, "Current Practices for
Multiple-Interface Hosts", RFC 6419, DOI 10.17487/RFC6419,
November 2011, .
[RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple
Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,
.
[RFC7323]Borman, D., Braden, B., Jacobson, V., and R. Scheffenegger,
Ed., "TCP Extensions for High Performance", RFC 7323, DOI
10.17487/RFC7323, September 2014, .
[RFC7822] Mizrahi, T. and D. Mayer, "Network Time Protocol Version 4
(NTPv4) Extension Fields", RFC 7822, DOI 10.17487/RFC7822,
March 2016, .
Authors' Addresses
Fei Song
Beijing Jiaotong University
Beijing, 100044
P.R. China
Email: fsong@bjtu.edu.cn
Hongke Zhang
Beijing Jiaotong University
Beijing, 100044
P.R. China
Email: hkzhang@bjtu.edu.cn
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H Anthony Chan
Huawei Technologies
5340 Legacy Dr. Building 3
Plano, TX 75024
USA
Email: h.a.chan@ieee.org
Anni Wei
Huawei Technologies
Xin-Xi Rd. No. 3, Haidian District
Beijing, 100095
P.R. China
Email: weiannig@huawei.com
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